You came all the way to San Francisco and you're going to let some stranger in a chef's hat cook your dungeness crab for you?
Pshaw. There's a little-known alternative, even among locals, that's far more fun: Zip down the coast half an hour to Half Moon Bay and pull into the scenic, fragrant, non-touristy Pillar Point Harbor. Roam happily, then make your way to whichever fisherman's selling crab that day, right off the boat. Chat, buy, drive, boil, eat; repeat as needed.
Call the Fishfone first to see what's running. Depends on season, availability, etc.